The present patent application is related to a patent application entitled "Electromagnetic Energy Diversion Device Responsive to the Power Density of the Energy" (formerly "Pellicle Laser Power Limiter") by David B. Cohn and Lorna C. Finman, filed on July 9, 1986 and assigned to Hughes Aircraft Company.
1. Field of the Invention
The present invention pertains to visible and infrared detectors and sensor systems. The specific focus of the invention is the protection of sensitive sensor components from the damaging effects of radiation from high-power laser sources.
2. Background Information
The field of optical engineering divides into two principal areas of practice. On one hand, light (that is, ultraviolet (UV), infrared (IR), and visible electromagnetic radiation) is generated, manipulated, and utilized for a myriad of imaging, diagnostic, and metrological purposes. On the other hand, the created and utilized light must be sensed by some arrangement of detectors for its full utility to be realized. When optics was still young as a discipline, the principal sensing system was either the human eye or a photographic emulsion. But as the field of optical science has burgeoned in recent years, ever more sensitive and accurate detectors have been required.
The techniques of semiconductor fabrication have produced extraordinarily accurate sensing devices. Focal plane arrays rely on photoconductive effects in semiconductor junctions to register incidence of light upon their surface. When combined in optical systems incorporating light-gathering apparatus, such as in a camera, focal plane arrays produce electrical signals which very accurately duplicate the image or light wavefront being detected. The resulting signals can then be operated upon by electrical signal processing equipment and be available for certain desired effects. The detection network described above forms the basis of television signal generation. Light detection systems are also central to the world's vast network of satellite cameras and mappers. Since focal plane arrays may be designed for specific wavelengths of light, a satellite mapper can detect the visible light from terrestrial weather, the infrared patterns of land and vegetation, or ultraviolet disturbances on the sun.
Laboratory uses for detectors are also manifold. The accuracy and flexibility afforded by optical instrumentation ensures the importance of reliable, sensitive detectors for industry and science.
The greatest strength of the new generation of detectors, i.e., their ability to accurately record light images at very low intensity, proves to be their greatest weakness. Inadvertent pulses of high-intensity light, focused upon these focal plane arrays by powerful optical elements, can temporarily saturate or permanently damage their thin and fragile semiconductor surfaces. The omnipresence of the laser in laboratory settings greatly increases the risk of accidental destruction of expensive precision detection equipment. In addition, military sensors (for instance, IR sensors used to see at night or in smoke-filled environments) are particularly vulnerable to unfriendly jamming and damage from laser sources. All detectors used to monitor low-level light intensities require some form of protection against careless or willful destruction.
Only recently has much effort been expended to create the needed protection methods for sensitive detectors. A variety of means for protection against high-intensity laser radiation have been proposed and investigated, some more esoteric than others. The simplest and most obvious protection device is a mechanical shutter. When the focal plane array detects some unacceptable rise in light intensity, a shutter closes over the aperture. The difficulty with this method is the intrinsically long risetime for moving the mass of the shutter into place. The laser radiation may damage the substrate within nanoseconds; by the time the shutter has closed off all further radiation, the sensor has been destroyed.
In order to improve the risetimes of the protection methods, various optical phenomena have been exploited to prevent harmful high-power light from striking the detector. For instance, certain gases under high pressure will strongly absorb radiation at a particular wavelength while transmitting all others. However, this method also has severe disadvantages. First, the high pressures involved can be dangerous, and, second, the method does not provide broadband protection from a variety of potential threats. Even when the gases do absorb hazardous laser light, the absorption is never total and a strong enough signal will eventually overpower this method of protection.
Similarly, absorbing optical coatings composed of dielectric and metallic films have been tried to absorb light of particular wavelengths. Presumably, all feasible laser threats would be catalogued, the most likely selected to protect against, and absorbing filters would be designed and constructed to eliminate only those wavelengths. Again, such band-stop filter techniques never totally prevent a strong light signal from reaching the detector surface. Moreover, the protected bands prevent desired radiation at those wavelengths from entering the detector, sharply reducing the signal-to-noise ratio for the sensor.
Several nonlinear optical effects have been exploited in attempting to protect sensor systems. Nonlinear defocusing in liquid crystal media, which causes divergence of high-power laser beams, can provide broadband protection against laser threats. However, it too has a rather long risetime for its protective effects to occur. The various techniques for nonlinear phase conjugation have also been proposed to defend detector arrays. In these approaches, a nonlinear medium pumped by sensor-based laser sources can very efficiently reflect backwards a potentially damaging high-power laser beam. Unfortunately, phase conjugation methods are extremely sensitive to wavelength, requiring that a threat be known very accurately in advance. In addition, as with the nonlinear defocusing effect, enormous power densities are required before this technique will work. Most nonlinear effects, while useful in other areas of optical science, are inadequate for wideband protection of delicate sensor systems.
Instead of preventing hazardous radiation from reaching the focal plane, attempts have been made to "harden" the detector array itself. In U.S. Pat. No. 4,117,329, Kruer et al. describe a technique for constructing detectors which are thermally connected to large heat sinks. The resulting configuration allows absorbed heat radiation to be efficiently carried away from the photoconductive material at the detector surface.
A truly practical and reliable means for protecting detector arrays from the damaging effects of stray high-power laser radiation without effecting great alterations to conventional sensor designs would constitute a major advance in the optical engineering field. Such an invention would satisfy a long-felt need experienced by the optical science community for over two decades. Producers of sensors and detectors could employ such an innovative device to conveniently defend costly and sensitive sensor components from all possible kinds of laser-induced damage. Such an invention would ideally be suited to operate in cooperation with a wide variety of sensing systems and detector arrays and would perform consistently and reliably over a wide range of operating conditions in various system applications.